The present invention relates to a method for curing a fluorine-containing polymer. In particular, the present invention relates to a method for curing a curable fluorine-containing polymer having functional groups on side chains, which is curable at room temperature.
Transparent fluororesins having a cyclic structure are disclosed as transparent fluororesins in U.S. Pat. No. 3,978,030, JP-B-5-49692, JP-A-5-117418, JP-A-63-238111, JP-A-63-238115, etc. However, those fluororesins are all thermoplastic resins and their heat resistance has its own limit.
U.S. Pat. Nos. 3,546,186 and 3,933,767 disclose a method for producing a crosslinked rubber by introducing a fluorinated cyano group to a side chain and trimerizing it.
However, transparent fluororesins, which can be cured at room temperature and additionally thermally cured, are not known.
One object of the present invention is to provide a method for curing a fluorine-containing polymer at room temperature to form a transparent fluororesin having better heat resistance than conventional transparent thermoplastic resins.
The above object is achieved by
a method for curing a fluorine-containing polymer having a structure of the formula: 
wherein X, Y and Z represent independently from each other a hydrogen atom, a fluorine atom, a chlorine atom, a bromine atom or an iodine atom, provided that at least one of X, Y and Z is a fluorine atom; R represents a straight or branched fluorinated alkylene group having 1 to 20 carbon atoms which may contain an oxygen atom; x and y represent mole percentages and x is from 1 to 100% by mole; A is xe2x80x94CN, xe2x80x94NCO, xe2x80x94COORxe2x80x2 in which Rxe2x80x2 is a hydrogen atom or an alkyl group having 1 to 10 carbon atoms, an acid anhydride group or an unsaturated hydrocarbon group; and M is a repeating unit derived from a copolymerizable monomer comprising treating said polymer with at least one compound selected from the group consisting of ammonia, diamines and polyol compounds and crosslinking said polymer through the side functional groups of said polymer, and
a method for curing a fluorine-containing polymer having a structure of the formula: 
wherein X, Y, Z, Xxe2x80x2, Yxe2x80x2 and Zxe2x80x2 represent independently from each other a hydrogen atom, a fluorine atom, a chlorine atom, a bromine atom or an iodine atom, provided that at least one of X, Y and Z is a fluorine atom; R1 and R2 represent independently each other a straight or branched fluorinated alkylene group having 1 to 20 carbon atoms which may contain an oxygen atom; z and w represent mole percentages and z is from 1 to 100% by mole; and E is an organic group other than xe2x80x94CN comprising curing said polymer in the presence of a base.
Among the monomers (III) constituting the structure (I) represented by the formula: 
wherein X, Y, Z, R and A are the same as defined above, a fluorine containing allyl ether nitrile of the formula (III) in which A is xe2x80x94CN and R is a fluorinated alkylene group containing an oxygen atom is a known compound and disclosed in JP-A-10-237130 together with the synthesis process thereof.
The monomer of the formula (II), in which A is a functional group other than xe2x80x94CN, that is, A being xe2x80x94NCO, xe2x80x94COORxe2x80x2, an acid anhydride group or an unsaturated hydrocarbon group, is a known compound or may be easily prepared by a known synthesis process.
Preferred examples of the acid anhydride group as the group A in the formula (I) include 
etc. and preferred examples of the unsaturated hydrocarbon group include xe2x80x94OCFxe2x95x90CF2, xe2x80x94CFxe2x95x90CF2, xe2x80x94CHxe2x95x90CH2, etc.
Preferred examples of the fluorinated alkylene group represented by R include xe2x80x94[CF2OCF(CF3)]axe2x80x94, xe2x80x94(CF2OCF2CF3)axe2x80x94 wherein a is a number of 1 to 4, xe2x80x94(CF2)bxe2x80x94 wherein b is a number of 1 to 20, etc.
The comonomer M which constitutes the structure of the formula (I) may be any monomer copolymerizable with the monomer (II). Preferred examples of the comonomer include vinylidene fluoride, tetrafluoroethylene, hexafluoropropylene, chlorotrifluoroethylene, vinyl fluoride, trifluoroethylene, tetrafluoropropylene, trifluoropropylene, ethylene, propylene, etc.
As described above, the polymer to be cured by the method of the present invention has at least one functional group selected from the group consisting of xe2x80x94CN, xe2x80x94NCO, xe2x80x94COORxe2x80x2, an acid anhydride group and an unsaturated hydrocarbon group in the molecule, preferably xe2x80x94CN (cyano group).
For example, J. Org. Chem., Vol. 32, 2237 describes that a cyano group readily reacts with ammonia to form an amidine or an imidoylamidine. The present invention utilizes this reaction to cure the fluorine-containing polymer of the formula (I).
Since the reaction of the cyano group with ammonia readily proceeds at room temperature, a colorless transparent cured product having elasticity can be obtained by simply allowing the polymer having the cyano group in contact with gaseous ammonia.
When A is xe2x80x94NCO, xe2x80x94COORxe2x80x2, an acid anhydride group or an unsaturated hydrocarbon group, the curing reaction proceeds by the addition of an alcohol or an amine to xe2x80x94NCO, amination through the reaction of xe2x80x94COORxe2x80x2 with an amine, amidation or imidation through the reaction of the acid anhydride with an amin, or the addition of an alcohol or an amine to the unsaturated hydrocarbon group.
Examples of a diamine to be used as a crosslinking agent include H2Nxe2x80x94Rxe2x80x3xe2x80x94NH2 wherein Rxe2x80x3 is an alkylene group having 1 to 20 carbon atoms, 
etc.
Examples of the polyol compound include 
wherein Rf is a fluoroalkyl group having 1 to 20 carbon atoms.
The amount of the crosslinking agent to be used is half an equivalent of the functional group A in the case of ammonia or an amine, or the hydroxyl group of the polyol is equivalent to the functional group A.
The cured product will form a triazine ring, etc. by further heating when the functional group A is a cyano group and the crosslinking agent is ammonia. Thus, the curing of the polymer further proceeds and the heat resistance of the cured product is improved.
The polymer of the formula (II) can be cured at room temperature in the presence of a base such as triethylamine, tributylamine, pyridine, etc.
In this case, the base may be compounded in the polymer, or allowed in contact with the cast film of the polymer.
The cured product obtained by such a method is also colorless and transparent. The hardness of the cured product may be freely adjusted by the adjustment of the amount of the base to be used. The cured product obtained in such a way will form a triazine ring by heating like the above case so that the curing of the polymer further proceeds and the heat resistance of the cured product is improved.
Examples of the fluorinated alkylene group represented by R1 or R2 in the formula (II) are the same as those of R.
Examples of the organic group other than xe2x80x94CN represented by E include the functional groups represented by A (except a cyano group), and also other organic groups such as straight or branched alkyl groups having 1 to 20 carbon groups in which a part or all of the hydrogen atoms may be substituted with a chlorine atom or a fluorine atom, or the alkyl chair may contain an oxygen atom.
The heating temperature is usually at least 100xc2x0 C., preferably at least 150xc2x0 C. In general, the heating temperature may not be higher than 300xc2x0 C.
The heating atmosphere is not limited. The heating may be carried out in an air or an atmosphere of an inert gas such as nitrogen.
The cured product obtained from the polymer according to the present invention may be used as a material for producing a sheath of an optical fiber, an antireflection film, an interlaminar insulation film, etc. in the electronic field or the optical field.